96 Liquid-gas and liquid-liquid interfaces
Substituting in the Dupre equation, the spreading coefficient can
be related to the work of adhesion and cohesion5 = Wow - 2yOA = Wow ~ W0ll (432)i.e. spreading occurs when the oil adheres to the water more strongly
than it coheres to itself.
Impurities in the oil phase (e.g. oleic acid in hexadecane) can
reduce y 0 w sufficiently to make 5 positive. Impurities in the aqueous
phase .normally reduce S, since -yWA is lowered more than y 0 w by the
impurity, especially if yOw is already low. Therefore, n-octane will
spread on a clean water surface but not on a contaminated surface.
The initial spreading coefficient does not consider the mutual
saturation of one liquid with another: for example, when benzene is
spread on water,Sinit /mNn-f^1 =72.8-(28.9 + 35.0) = +8.9but when the benzene and water have had time to become mutually
saturated, -yWA is reduced to 62.4 mN m~* and -yow to 28.8 mN m"^1 ,
so that
Sfma,/mNm != 62.4-(28.8+ 35.0) = -1.4The final state of the interface is now just unfavaourable for
spreading. This causes the initial spreading to be stopped, and can
even result in the film retracting slightly to form very flat lenses, the
rest of the water surface being covered by a monolayer of benzene.
Similar considerations apply to the spreading of a liquid such as
«-hexanoI on water (Figure 4.17): '(^5) init /mNnT^1 =72.8-(24.8 + 6.8) = +41.2
Sfinal / inNm'^1 = 28.5 - (24.7 + 6.8) = -3.0
Monomolecular films^54 57
Many insoluble substances, such as long-chain fatty acids and
alcohols, can (with the aid of suitable solvents) be spread on to a
water surface, and if space permits will form a surface film one